How can I add elements to arrays without overwriting them?


This Content is from Stack Overflow. Question asked by Chris_Len

I have an issue with saving items into an array in JavaScript (or Next.js). At the beginning I have an empty array. Then a session is started, and I check if the session is empty or not. If it is empty, I write a response of a get-request into a variable. At the same time I also write the response of the get request into the local storage.
Basically I now want to write the value of the local storage into the array with the push-method.
In the next step my program should realize, that the session is not empty anymore and now the new value of the get-request should be stored into the local storage and at the same time it should be added as a new element of the variable. The old elements of the array should not be overwritten. I have now tried a couple of things, but I could not get it to work.

Does anybody have a hint for me, how to realize this problem? I would be very thankful, if anybody could help me on this one 😊

  'products?include=' + mainId,
  addOrViewCartConfig // a config which is not important for the question
.then((response) => {
  // Successful request
  console.log('yes, it worked');
  console.log('response', response);

  if (isEmpty(storedSession)) {
    var localCart = [];
    console.log('session', storedSession);
    console.log('localCart1', localCart);
    // return localCart;
  console.log('localCart2', localCart);
  console.log('session1', storedSession);

  if (!localCart.indexOf(parseInt(localStorage.getItem('ID')))) {
  } else {
    alert('The item you are trying to add is already in your cart!');
    return false;
  console.log('localCart3', localCart);

.catch((error) => {
  //Invalid request, for 4xx and 5xx statuses


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